[FIX] server: graceful shutdown must send SIGINT instead of SIGTERM to workers

Workers do not specifically handle SIGTERM so it can be used to
force quit them, but the graceful shutdown is triggered by SIGINT.

Closes #6738

openerp/service/server.py

@@ -613,7 +613,7 @@ class PreforkServer(CommonServer):
             _logger.info("Stopping gracefully")
             limit = time.time() + self.timeout
             for pid in self.workers.keys():
-                self.worker_kill(pid, signal.SIGTERM)
+                self.worker_kill(pid, signal.SIGINT)
             while self.workers and time.time() < limit:
                 self.process_zombie()
                 time.sleep(0.1)